Recently, there is a growing trend toward reducing the basis weight of coated printing papers to meet demands for lighter prints for the purposes of saving resources, transportation costs and the like. As the basis weight decreases, the opacity generally decreases, but if the opacity is low, images printed on one side appear on the other side, thereby reducing the value of the prints. Thus, it would be desirable to maintain high opacity at low basis weight.
In addition to these demands, there has recently been a high demand for communicating the contents of prints in a visually impressive manner by frequently using photographs and graphics and further colorizing them. In such a demand, brightness is important. This is because if brightness is low, printed images appear darker than intended so that their contents are communicated less impressively. However, brightness and opacity are normally contradictory to each other, and there is a tendency that when brightness is high, opacity is low, while when opacity is high, brightness is low. Thus, it is necessary to strike a balance between brightness and opacity.
As to brightness, not only brightness determined by conventional methods but also “brightness variation” is important. As used herein, brightness variation is defined as the standard deviation of brightness in microscopic areas. Prints having significant brightness variation lose their value even if they have high brightness because they are poor in surface appearance and the variation is further emphasized especially in halftone dot areas of the prints. Generally, brightness variation is especially prominent when there is a great difference in brightness between the base paper and the coating layer. Thus, attempts were made to reduce brightness variation by using a base paper having somewhat high brightness to reduce the difference in brightness between the base paper and the coating layer or the like, but the high-brightness base paper resulted in low opacity.
As indicated above, it is difficult to prepare prints having high levels of both opacity and brightness as well as improved brightness variation at low basis weight with good runnability by conventionally known methods.